Contamination of tidal sediment with oil and the development of microbial populations associated with oil was simulated in aquaria in the laboratory. Even distribution of a thin viscous mixture of oil and natural sediment over pristine sediment was a controllable method to achieve development of bioactive horizons (mat-like stratification) attributable to distinct microbial activities (phototrophic, aerobic, anaerobic). Microsensor measurements and observations of color changes showed that the addition of oil significantly promoted subsurface oxygen consumption and microbial sulfate reduction. There was so far no clear indication that cyanobacterial growth was stimulated by the added oil. Sediment microcosms as established in the present study are promising systems for the future study of the long-term fate of oil on and in marine sediments and of the significance of bacterial guilds and environmental parameters (light, oxygen, fertilization, etc.) for biodegradation of oil.